High efficiency light sources, and in particular solid-state light sources such as LEDs, are increasingly replacing incandescent light sources in a wide range of applications. In contrast to incandescent (filament) lamps, such light sources generally are not directly compatible with AC power supplies and in particular the mains power available in most countries.
Conventionally, to power such light sources, an AC power source is converted into DC, typically by means of a switched mode power converter. However this adds complexity and cost to the overall system; there is an increasing interest in so-called “direct-to-mains” driving of LEDs. Since, in a mains supply both the current and voltage vary, a high voltage is available only during a part of the mains cycle. So, in order to be able to drive at least some of the LEDs across more of the mains cycle, the LEDs are grouped into strings of series-connected LEDs. Once the mains voltage is sufficient to power a first string of LEDs, that string is connected to the supply. As the supply voltage rises a second string of LEDs is switched to be in series with the first string. Thereafter at a still higher mains voltage a third string is connected in series with both the first and second string, and so on until all of the strings are connected in series, for the high-voltage part of the AC supply cycle. Then, as the supply voltage starts to fall, strings are sequentially switched out of the series arrangement until just one string is connected across the supply. Towards the end of the cycle—and the beginning of the next cycle—when the voltage is insufficient to illuminate even one string, no strings are supplied. This is generally termed a “linear LED driver”, and it shown schematically in FIG. 1. The figure shows an AC supply voltage, each half-cycle of which is split into several segments or regions. During the regions labelled 1-4, one, two, three and then four strings are connected respectively; in region 5 only three strings are connected; in region 6 just two strings are connected and in region 7, one string only is connected. It will be appreciated that the stepped current profile shown in FIG. 1 is for illustration purposes; in general, the current will more closely follow the mains voltage shape, for higher efficiency, although in principle, the current could be linear, or constant,
It is generally preferred to provide a constant current supply to a string of LEDs rather than a constant voltage, since, among other reasons, the output light flux more closely follows the current through the device than the voltage across it. The driver and control circuit for such an arrangement generally includes a current source and one or more switches to direct the current to the relevant series combination of strings, which combination thus changes during the supply cycle. During the parts of the supply cycle where in the supply voltage is not exactly matched to the LEDs strings, the excess voltage is dropped in the driver. This voltage can correspond to a significant power which is thus dissipated as heat energy in the driver and/or controller.